There is an increasing demand for three dimensional (3D) video or image capture, as well as increasing demand for object tracking or object scanning. Thus, the interest in 3D imaging is not simply to sense direction, but also depth. Longer wavelength signals (such as radar) have wavelengths that are too long to provide the sub-millimeter resolution required for smaller objects and for recognition of finger gestures and facial expressions. LIDAR (light detection and ranging) systems use optical wavelengths, and can provide finer resolution. A basic LIDAR system includes one or more light sources and photodetectors, a means of either projecting or scanning the light beam(s) over the scene of interest, and one or more control systems to process and interpret the data. Scanning or steering the light beam traditionally relies on precision mechanical parts, which are expensive to manufacture, and are bulky and consume a lot of power.
A known issue with ranging systems based on reflected signals is that detection relies on collecting reflected signal energy from diffuse surfaces. It is known that scattered optical power degrades as the square of the distance, and since the probe light scatters in all directions off the target object this leads to limitations on the range over which a reflected signal can be detected with sufficient signal-to-noise ratio (SNR). The range can be improved by increasing the size of the receiver collection optics, which allows for capturing more of the reflected signal, but the increased size limits application of the device. Collection of optical pulses over a long period of time together with techniques such as averaging or narrowband filtering can also be used to increase SNR, but such techniques require increased dwell time on a portion of the field of view, thus limiting the frame rate with which the system can monitor the target. Additionally, the periodic nature of the pulse patterns leads to ambiguity in the timing of the returned signal, which increases ambiguity with respect to the precise range to the target.
Finally, recording the range to the target with high precision is dependent on the precision with which the timing of returned pulses can be recorded. The timing with which the returned pulses can be recorded in turn either requires pulse widths comparable to the depth resolution, or requires that the pulses be recovered with sufficient SNR to distinguish their peak from the rest of the pulse. This tradeoff between pulse width, dwell time, and SNR is known as the Cramer-Rao lower bound. Thus, obtaining a usable echo in implementation requires a certain minimum energy to be transmitted in order to maintain sufficient signal to noise ratio for detection of the target.
To capture sufficient reflected power to detect objects at several meters distance at even VGA (video graphics array) resolution and frame rates using compact collection optics traditionally requires the use of transmit powers that pose eye-safety risks. In addition to posing risks for eye safety, such high-power transmit or output power is incompatible with portable or wearable devices. However such systems are typically not easily integrated into a small form-factor together with a means of scanning the optical beam across the field of view.
Descriptions of certain details and implementations follow, including a description of the figures, which may depict some or all of the embodiments described below, as well as discussing other potential embodiments or implementations of the inventive concepts presented herein.